My son's favorite is "dumb as a can of soup". For a long time, I used "He has the intelligence of an amoeba".
Back in the day, "If BS was music, you'd be a brass band". Or "Here comes f**kwit of the month".
Two of my favorites are responses:
Churchill and Bessie Braddock:
Braddock: "Winston, you're drunk!"
Churchill: " Madam, you're ugly, But I'll be sober in the morning".
Disraeli and an opposition MP:
Sir, you will either die on the gallows or of some unspeakable disease.'
That depends, Sir,' said Disraeli, 'whether I embrace your policies or your mistress.”
Have at it.